The present invention relates to optical communication systems and more particularly to systems and methods for multiplexing to provide optical pump energy.
Dense wavelength division multiplexing (DWDM) systems are evolving to both increase the distances over which DWDM signals may travel without regeneration and also to expand data carrying capacity by increasing the number of channels. To support this evolution, DWDM amplification technology is increasingly relying on Raman amplifiers even to the exclusion of Erbium-doped fiber amplifiers (EDFAs) due to the greater bandwidth of the Raman amplifiers. Distributed Raman amplifiers (DRAs) provide amplification within transmission spans while lumped Raman amplifiers (LRAs) are positioned between spans to provide further all-optical amplification as necessary. Both types of Raman amplifiers require that coherent pump energy from laser sources be pumped into a fiber through which the signal to be amplified propagates. This all-Raman approach greatly extends the available bandwidth over systems that rely all or in part on EDFAs.
Further improvement in system bandwidth and performance requires further improvement of the available bandwidth and the gain flatness over that bandwidth for both the LRAs and the DRAs. This can be accomplished by using multiple Raman pumps at different wavelengths. By appropriately selecting pump wavelengths, optimal gain flatness and signal to noise ratio may be achieved. Gain flatness is important so that all of the numerous channels accommodated by the wider bandwidth remain within the dynamic range of the optical receivers used to recover the transmitted information. Difficulty arises in multiplexing together multiple pump signals prior to injection into the transmission fiber.
One approach is to use as many couplers as necessary to combine the outputs of multiple lasers. To combine N lasers requires Nxe2x88x921 couplers. Loss introduced by the couplers will then be (Nxe2x88x921)*3 dB. The pump power that can be practically generated is limited by cost, safety, and reliability concerns. Given these constraints on pump power and the loss introduced by the use of couplers, this approach can only be used to combine a relatively small number of pump outputs. The desired improvement in bandwidth cannot be provided using this approach.
An alternative approach is to employ interferential filters to multiplex multiple pumps. FIG. 1 depicts a cascaded configuration of multiple interferential filters 102. For each Raman pump wavelength, there are two orthogonally polarized lasers outputting to a polarization beam combiner (PBC) 104. The use of the PBCs reduces the degree of polarization of the multiplexed Raman pump signal to avoid polarization dependent gain (PDG) effects. Again, there is a relatively high loss with the loss for the nth filter being 0.7+0.7*(nxe2x88x921) with a maximum loss of 0.7*(Nxe2x88x921) where N is the number of pump wavelengths. Again, the high loss reduces the bandwidth that can be achieved due to the constraints on laser power.
FIG. 2 depicts an alternative configuration of interferential filters 102, a tree configuration. Here the loss is 0.7*log2 (N) where N is the number of pump wavelengths and is a power of 2. Although the tree configuration reduces the loss introduced, there is still a problem of tightly controlling the characteristics of numerous components. The interferential filters have very strict requirements, e.g., a flat response over 50 nm or more in both the transmitted and reflected spectra and a 3-dB roll off at 200 GHz from the pass band edge. Also the polarization extinction ratio of the large numbers of PBCs must be strictly controlled. Thus, even when acceptable laser powers may be used, cost and complexity are still higher than desired.
What is needed are systems and methods for multiplexing Raman pump energy sources to provide greatly improved amplification bandwidth at low cost while requiring no more laser power than is practical.
By virtue of one embodiment of the present invention, optical amplification bandwidth is improved by providing an economical and practical way of multiplexing together multiple Raman pump energy sources(at disparate wavelengths. An arrayed waveguide grating (AWG) may be used to implement a double multiplexer. The insertion loss of the AWG is very low, allowing a reduction in the necessary power. Also, a single AWG may be used to combine numerous pump wavelengths.
According to a first aspect of the present invention, a method for providing pump energy for optical amplification includes: directing optical energy from a first set of coherent optical energy sources having disparate wavelengths to a first set of corresponding input ports of an AWG, and obtaining a first multi-wavelength pump signal from a first output port of the AWG disposed to receive optical energy from the first set of input ports.
According to a second aspect of the present invention, apparatus for providing pump energy for optical amplification includes: a first set of coherent optical energy sources having disparate output wavelengths and an AWG, wherein the outputs of the first set of coherent optical energy sources are coupled to corresponding input ports of a first set of input ports of the AWG. A first multi-wavelength pump signal is obtained from a first output port of the AWG, the first output port being disposed to receive optical energy from the first set of input ports.
Further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings.